Type 2 immunity is controlled by IL-4/IL-13 expression in hematopoietic non-eosinophil cells of the innate immune system

Dupilumab efficacy and safety in adults with uncontrolled persistent asthma despite use of medium-to-high-dose inhaled corticosteroids plus a long-acting β2 agonist: a randomised double-blind placebo-controlled pivotal phase 2b dose-ranging trial

BACKGROUND

Dupilumab, a fully human anti-interleukin-4 receptor α monoclonal antibody, inhibits interleukin-4 and interleukin-13 signalling, key drivers of type-2-mediated inflammation. Adults with uncontrolled persistent asthma who are receiving medium-to-high-dose inhaled corticosteroids plus a long-acting β2 agonist require additional treatment options as add-on therapy. We aimed to assess the efficacy and safety of dupilumab as add-on therapy in patients with uncontrolled persistent asthma on medium-to-high-dose inhaled corticosteroids plus a long-acting β2 agonist, irrespective of baseline eosinophil count.

METHODS

We did this randomised, double-blind, placebo-controlled, parallel-group, pivotal phase 2b clinical trial at 174 study sites across 16 countries or regions. Adults (aged ≥18 years) with an asthma diagnosis for 12 months or more based on the Global Initiative for Asthma 2009 Guidelines receiving treatment with medium-to-high-dose inhaled corticosteroids plus a long-acting β2 agonist were eligible for participation. Patients were randomly assigned (1:1:1:1:1) to receive subcutaneous dupilumab 200 mg or 300 mg every 2 weeks or every 4 weeks, or placebo, over a 24-week period. The primary endpoint was change from baseline at week 12 in forced expiratory volume in 1 s (FEV1 in L) in patients with baseline blood eosinophil counts of at least 300 eosinophils per μL assessed in the intention-to-treat population. Safety outcomes were assessed in all patients that received at least one dose or part of a dose of study drug. This trial is registered at ClinicalTrials.gov, number NCT01854047, and with the EU Clinical Trials Register, EudraCT number 2013-000856-16.

FINDINGS

769 patients (158 in the placebo group and 611 in the dupilumab groups) received at least one dose of study drug. In the subgroup with at least 300 eosinophils per μL, the greatest increases (200 mg every 2 weeks, p=0·0008; 300 mg every 2 weeks, p=0·0063) in FEV1 compared with placebo were observed at week 12 with doses every 2 weeks in the 300 mg group (mean change 0·39 L [SE 0·05]; mean difference 0·21 [95% CI 0·06-0·36; p=0·0063]) and in the 200 mg group (mean change 0·43 L [SE 0·05]; mean difference 0·26 [0·11-0·40; p=0·0008]) compared with placebo (0·18 L [SE 0·05]). Similar significant increases were observed in the overall population and in the fewer than 300 eosinophils per μL subgroup (overall population: 200 mg every 2 weeks, p<0·0001; 300 mg every 2 weeks, p<0·0001; <300 eosinophils per μL: 200 mg every 2 weeks, p=0·0034; 300 mg every 2 weeks, p=0·0086), and were maintained to week 24. Likewise, dupilumab every 2 weeks produced the greatest reductions in annualised rates of exacerbation in the overall population (70-70·5%), the subgroup with at least 300 eosinophils per μL (71·2-80·7%), and the subgroup with fewer than 300 eosinophils per μL (59·9-67·6%). The most common adverse events with dupilumab compared with placebo were upper respiratory tract infections (33-41% vs 35%) and injection-site reactions (13-26% vs 13%).

INTERPRETATION

Dupilumab increased lung function and reduced severe exacerbations in patients with uncontrolled persistent asthma irrespective of baseline eosinophil count and had a favourable safety profile, and hence in addition to inhaled corticosteroids plus long-acting β2-agonist therapy could improve the lives of patients with uncontrolled persistent asthma compared with standard therapy alone.

FUNDING

Sanofi-Genzyme and Regeneron Pharmaceuticals.

Th2 and eosinophil responses suppress inflammatory arthritis

Th2–eosinophil immune responses are well known for mediating host defence against helminths. Herein we describe a function of Th2–eosinophil responses in counteracting the development of arthritis. In two independent models of arthritis, Nippostrongylus brasiliensis infection leads to Th2 and eosinophil accumulation in the joints associated with robust inhibition of arthritis and protection from bone loss. Mechanistically, this protective effect is dependent on IL-4/IL-13-induced STAT6 pathway. Furthermore, we show that eosinophils play a central role in the modulation of arthritis probably through the increase of anti-inflammatory macrophages into arthritic joints. The presence of these pathways in human disease is confirmed by detection of GATA3-positive cells and eosinophils in the joints of rheumatoid arthritis patients. Taken together, these results demonstrate that eosinophils and helminth-induced activation of the Th2 pathway axis effectively mitigate the course of inflammatory arthritis.

Type 2 immune responses are viewed as opposites of Type 1 and 17 responses. Here the authors show that activation of Type 2 immunity by helminth infection counteracts the development of inflammatory arthritis, a type 17-mediated pathology, via IL-4/IL-13- STAT6 signalling and eosinophil activation.

Depletion of major pathogenic cells in asthma by targeting CRTh2

Eosinophilic inflammation and Th2 cytokine production are central to the pathogenesis of asthma. Agents that target either eosinophils or single Th2 cytokines have shown benefits in subsets of biomarker-positive patients. More broadly effective treatment or disease-modifying effects may be achieved by eliminating more than one inflammatory stimulator. Here we present a strategy to concomitantly deplete Th2 T cells, eosinophils, basophils, and type-2 innate lymphoid cells (ILC2s) by generating monoclonal antibodies with enhanced effector function (19A2) that target CRTh2 present on all 4 cell types. Using human CRTh2 (hCRTh2) transgenic mice that mimic the expression pattern of hCRTh2 on innate immune cells but not Th2 cells, we demonstrate that anti-hCRTh2 antibodies specifically eliminate hCRTh2⁺ basophils, eosinophils, and ILC2s from lung and lymphoid organs in models of asthma and Nippostrongylus brasiliensis infection. Innate cell depletion was accompanied by a decrease of several Th2 cytokines and chemokines. hCRTh2-specific antibodies were also active on human Th2 cells in vivo in a human Th2-PBMC-SCID mouse model. We developed humanized hCRTh2-specific antibodies that potently induce antibody-dependent cell cytotoxicity (ADCC) of primary human eosinophils and basophils and replicated the in vivo depletion capacity of their murine parent. Therefore, depletion of hCRTh2⁺ basophils, eosinophils, ILC2, and Th2 cells with h19A2 hCRTh2-specific antibodies may be a novel and more efficacious treatment for asthma.

Role of basophils in protective immunity to parasitic infections

Basophils have been recognized as important players for protective immunity against a variety of different endo- and ectoparasites. Although basophils represent a relatively rare and short-lived cell type, they produce large quantities of effector molecules including histamine, cytokines, chemokines, and lipid mediators which promote type 2 immune responses. Basophils can be activated either directly by parasite-derived factors or indirectly by recognition of parasite-derived antigens via IgE bound to its high-affinity receptor FcεRI on the cell surface. Many parasitic infections cause expansion and tissue recruitment of basophils, but the role of basophils for protective immunity remains poorly understood. The development of basophil-deficient mouse models over the past few years makes it possible to study their contributions in various infections. We review here the current knowledge regarding the role of basophils for protective or immunomodulatory functions of basophils mainly during infections of mice with protozoan parasites, helminths, and ectoparasites.

Cytokine expression by invariant natural killer T cells is tightly regulated throughout development and settings of type-2 inflammation

Invariant natural killer T (iNKT) cells produce cytokines interleukin-4 (IL-4) and IL-13 during type-2 inflammatory responses. However, the nature in which iNKT cells acquire type-2 cytokine competency and the precise contribution of iNKT cell-derived IL-4 and IL-13 in vivo remains unclear. Using IL-13-reporter mice to fate-map cytokine-expressing cells in vivo, this study reveals that thymic iNKT cells express IL-13 early during development, and this IL-13-expressing intermediate gives rise to mature iNKT1, iNKT2, and iNKT17 subsets. IL-4 and IL-13 reporter mice also reveal that effector iNKT2 cells produce IL-4 but little IL-13 in settings of type-2 inflammation. The preferential production of IL-4 over IL-13 in iNKT2 cells results in part from their reduced GATA-3 expression. In summary, this work helps integrate current models of iNKT cell development, and further establishes non-coordinate production of IL-4 and IL-13 as the predominant pattern of type-2 cytokine expression among innate cells in vivo.

Tuft-cell-derived IL-25 regulates an intestinal ILC2-epithelial response circuit

Parasitic helminths and allergens induce a type 2 immune response leading to profound changes in tissue physiology, including hyperplasia of mucus-secreting goblet cells and smooth muscle hypercontractility. This response, known as 'weep and sweep', requires interleukin (IL)-13 production by tissue-resident group 2 innate lymphoid cells (ILC2s) and recruited type 2 helper T cells (TH2 cells). Experiments in mice and humans have demonstrated requirements for the epithelial cytokines IL-33, thymic stromal lymphopoietin (TSLP) and IL-25 in the activation of ILC2s, but the sources and regulation of these signals remain poorly defined. In the small intestine, the epithelium consists of at least five distinct cellular lineages, including the tuft cell, whose function is unclear. Here we show that tuft cells constitutively express IL-25 to sustain ILC2 homeostasis in the resting lamina propria in mice. After helminth infection, tuft-cell-derived IL-25 further activates ILC2s to secrete IL-13, which acts on epithelial crypt progenitors to promote differentiation of tuft and goblet cells, leading to increased frequencies of both. Tuft cells, ILC2s and epithelial progenitors therefore comprise a response circuit that mediates epithelial remodelling associated with type 2 immunity in the small intestine, and perhaps at other mucosal barriers populated by these cells.

T cells are the critical source of IL-4/IL-13 in a mouse model of allergic asthma

BACKGROUND

IL-4 and IL-13 play a crucial role during allergic asthma. Both cytokines can be produced by T cells and a variety of cell types of the innate immune system. The relative contribution of T-cell-derived vs innate IL-4/IL-13 for allergic inflammation and airway hyperreactivity remains unclear.

METHODS

We compared the severity of OVA/alum-induced allergic lung inflammation in WT BALB/c mice to mice that lack expression of IL-4/IL-13 only in T cells (4-13Tko) or in all cell types (4-13ko).

RESULTS

T-cell-derived IL-4/IL-13 was required for IgG1 and IgE production, recruitment of eosinophils and basophils to the lung, goblet cell hyperplasia, expression of Muc5ac, Clca3, and RELMβ, differentiation of alternatively activated macrophages, and airway hyperreactivity. Interestingly, ILC2 recruitment to the lung occurred independently of T-cell-derived IL-4/IL-13 but was diminished in the absence of IL-4/IL-13 from all cell types. Thus, the number of IL-4/IL-13-competent ILC2s did not correlate with the severity of lung pathology.

CONCLUSIONS

Th2 cells appear to be the critical IL-4/IL-13-expressing cell type for the induction of allergic airway inflammation and airway hyperreactivity. The translational perspective of our results indicates that inhibition or reprogramming of Th2 cells may be very effective for the treatment of allergic asthma.

Administration of naloxone in combination with recombinant Plasmodium vivax AMA-1 in BALB/c mice induces mixed Th1/Th2 immune responses

Naloxone (NLX) has the ability to shift the immune response to a Th1 profile. Therefore, the adjuvant efficacy of NLX with recombinant P. vivax apical membrane antigen-1(rPvAMA-1) in BALB/c mice was evaluated. Mice were immunized subcutaneously with purified rPvAMA-1 formulated with NLX (doses of 5 mg/kg body weight) alone or in combination with IFA. A significant increase in anti-PvAMA-1 IgG antibody after the second boost (mean OD₄₉₀  = 2·08 and 2·17, in groups received, rPvAMA-1/NLX and rPvAMA-1/NLX/IFA, respectively) was detected. IgG1 and IgG2b were the predominant isotypes in all immunized mouse groups. In immunized mice with rPvAMA-1/NLX (mean: 1036 pg/mL) and with rPvAMA-1/NLX/IFA (mean: 1024 pg/mL), IFN-γ was elicited in response to rPvAMA-1 after the second boost. No detectable IL-4 secretion was determined in all tested groups. In conclusion, the administration of NLX alone or NLX/IFA with rPvAMA-1 in BALB/c mice, which induced mixed Th1/Th2 immune responses, was comparable with that of the same recombinant antigen with CFA/IFA adjuvant. The results indicate that NLX alone may possibly not be considered as a potent Th1 adjuvant in PvAMA-1-based vaccine. However, in order to modulate immune responses from mixed Th1/Th2 to strong and protective Th1 response, further study is warranted on combination of NLX with other adjuvants such as CpG motifs or MPL in proper vaccine formulation. Additionally, dose-response study is necessary to determine the effect of different doses of antigen combined with NLX (at various doses) in Balb/c mice.

The messenger between worlds: the regulation of innate and adaptive type-2 immunity by innate lymphoid cells

Although type-2 immune responses evolved primarily to defend against extracellular helminths, in part through the co-opting of tissue repair and remodeling mechanisms, they are often inappropriately directed towards relatively innocuous allergens resulting in conditions including asthma, allergic rhinitis, food allergy, and atopic dermatitis. The recent discovery of group 2 innate lymphoid cells (ILC2) has increased our understanding of the initiation of these responses and the roles played by CD4(+) T helper (Th) 2 cells in their modulation. This review focuses on the important messenger role of ILC2 in translating epithelial-derived alarmins into downstream adaptive type-2 responses via dendritic cells and T cells, with special emphasis on their roles in allergic disease.

Group 2 innate lymphoid cells in disease

Group 2 innate lymphoid cells (ILC2) are now recognized as an important innate source of type-2 effector cytokines. Although initially associated with mucosal tissues, it is clear that ILC2 are present in diverse anatomical locations. The function of ILC2 at these sites is equally varied, and although ILC2 represent a relatively minor population, they are fundamentally important regulators of innate and adaptive immune processes. As such, there is much interest to understand the role of ILC2 in diseases with a type-2 inflammatory component. This review explores the known roles of ILC2 in disease, and the diseases that show associations or other strong evidence for the involvement of ILC2.

Maintenance of Immune Homeostasis through ILC/T Cell Interactions

Innate lymphoid cells (ILCs) have emerged as a new family of immune cells with crucial functions in innate and adaptive immunity. ILC subsets mirror the cytokine and transcriptional profile of CD4⁺ T helper (T_H) cell subsets. Hence, group 1 (ILC1), group 2 (ILC2), and group 3 (ILC3) ILCs can be distinguished by the production of T_H1, T_H2, and T_H17-type cytokines, respectively. Cytokine release by ILCs not only shapes early innate immunity but can also orchestrate T_H immune responses to microbial or allergen exposure. Recent studies have identified an unexpected effector function of ILCs as antigen presenting cells. Both ILC2s and ILC3s are able to process and present foreign antigens (Ags) via major histocompatibility complex class II, and to induce cognate CD4⁺ T cell responses. In addition, Ag-stimulated T cells promote ILC activation and effector functions indicating a reciprocal interaction between the adaptive and innate immune system. A fundamental puzzle in ILC function is how ILC/T cell interactions promote host protection and prevent autoimmune diseases. Furthermore, the way in which microenvironmental and inflammatory signals determine the outcome of ILC/T cell immune responses in various tissues is not yet understood. This review focuses on recent advances in understanding the mechanisms that coordinate the collaboration between ILCs and T cells under homeostatic and inflammatory conditions. We also discuss the potential roles of T cells and other immune cells to regulate ILC functions and to maintain homeostasis in mucosal tissues.

Inflammatory group 2 innate lymphoid cells

Group 2 innate lymphoid cells (ILC2 cells) are able to produce type 2 cytokines and to mediate type 2 immune protection and tissue homeostasis. ILC2 cells have often been considered to be a single set of cells that respond to IL-33 and/or IL-25. Recent evidence now indicates that ILC2 cells can be grouped into two distinct subsets: homeostatic or natural ILC2s (nILC2 cells); and inflammatory ILC2 cells (iILC2 cells). nILC2 cells reside in barrier tissues and primarily respond to IL-33. They play critical roles not only in immune protection but also in tissue repair and beige fat biogenesis. iILC2 cells are not present in peripheral tissues in the steady state but can be elicited at many sites by helminth infection or IL-25 treatment. IL-25-elicited ilLC2 cells act as transient ILC progenitors with multipotency. They can be mobilized by distinct types of infections to develop into nILC2-like or ILC3-like cells, functioning in corresponding immune responses. The demonstration of the existence of iILC2 cells adds to our understanding of the complexity of ILC2 biology and makes necessary an analysis of the relationship between nILC2 cells and iILC2 cells.

Interleukin-33 and Interferon-γ Counter-Regulate Group 2 Innate Lymphoid Cell Activation during Immune Perturbation

Group 2 innate lymphoid cells (ILC2s) and regulatory T (Treg) cells are systemically induced by helminth infection but also sustain metabolic homeostasis in adipose tissue and contribute to tissue repair during injury. Here we show that interleukin-33 (IL-33) mediates activation of ILC2s and Treg cells in resting adipose tissue, but also after helminth infection or treatment with IL-2. Unexpectedly, ILC2-intrinsic IL-33 activation was required for Treg cell accumulation in vivo and was independent of ILC2 type 2 cytokines but partially dependent on direct co-stimulatory interactions via ICOSL-ICOS. IFN-γ inhibited ILC2 activation and Treg cell accumulation by IL-33 in infected tissue, as well as adipose tissue, where repression increased with aging and high-fat diet-induced obesity. IL-33 and ILC2s are central mediators of type 2 immune responses that promote tissue and metabolic homeostasis, and IFN-γ suppresses this pathway, likely to promote inflammatory responses and divert metabolic resources necessary to protect the host.

The differential expression of IL-4 and IL-13 and its impact on type-2 immunity

Allergic disease represents a significant global health burden, and disease incidence continues to rise in urban areas of the world. As such, a better understanding of the basic immune mechanisms underlying disease pathology are key to developing therapeutic interventions to both prevent disease onset as well as to ameliorate disease morbidity in those individuals already suffering from a disorder linked to type-2 inflammation. Two factors central to type-2 immunity are interleukin (IL)-4 and IL-13, which have been linked to virtually all major hallmarks associated with type-2 inflammation. Therefore, IL-4 and IL-13 and their regulatory pathways represent ideal targets to suppress disease. Despite sharing many common regulatory pathways and receptors, these cytokines perform very distinct functions during a type-2 immune response. This review summarizes the literature surrounding the function and expression of IL-4 and IL-13 in CD4+ T cells and innate immune cells. It highlights recent findings in vivo regarding the differential expression and non-canonical regulation of IL-4 and IL-13 in various immune cells, which likely play important and underappreciated roles in type-2 immunity.

T helper 2 (Th2) cell differentiation, type 2 innate lymphoid cell (ILC2) development and regulation of interleukin-4 (IL-4) and IL-13 production

Interleukin-4 (IL-4), IL-5 and IL-13, the signature cytokines that are produced during type 2 immune responses, are critical for protective immunity against infections of extracellular parasites and are responsible for asthma and many other allergic inflammatory diseases. Although many immune cell types within the myeloid lineage compartment including basophils, eosinophils and mast cells are capable of producing at least one of these cytokines, the production of these "type 2 immune response-related" cytokines by lymphoid lineages, CD4 T helper 2 (Th2) cells and type 2 innate lymphoid cells (ILC2s) in particular, are the central events during type 2 immune responses. In this review, I will focus on the signaling pathways and key molecules that determine the differentiation of naïve CD4 T cells into Th2 cells, and how the expression of Th2 cytokines, especially IL-4 and IL-13, is regulated in Th2 cells. The similarities and differences in the differentiation of Th2 cells, IL-4-producing T follicular helper (Tfh) cells and ILC2s as well as their relationships will also be discussed.

Type-2 innate lymphoid cells in human allergic disease

PURPOSE OF REVIEW

Recent decades have seen allergic diseases become endemic in a number of developed countries. Understanding the inflammatory processes that dictate these allergic responses is therefore important.

RECENT FINDINGS

Critical to many allergic responses is the inappropriate release of the type-2 immune-regulatory cytokines: interleukin-4, interleukin-5, interleukin-9, and interleukin-13. The study of these inflammatory mediators has led directly to the development of two new asthma treatments: anti-interleukin-5 and anti-interleukin-13. Until recently, T helper 2 cells were considered to be the major cellular source of type-2 cytokines; however, a paradigm shift occurred with the discovery of a novel population, type-2 innate lymphoid cells (ILC2s), that can produce huge levels of type-2 cytokines and are sufficient to induce allergy in mice. This discovery raises interesting questions about how innate and adaptive type-2 immunity might interact to induce relapsing and remitting episodes of allergy in patients.

SUMMARY

It is essential that alongside the mechanistic investigation using model organisms, the roles of ILC2s in human disease be explored. Here, we discuss how ILC2 traits, discovered in mouse models, have informed research in humans and how newly identified human ILC2 pathways might provide potential therapeutic benefits in the future.

Re-defining the unique roles for eosinophils in allergic respiratory inflammation

The role of eosinophils in the progression and resolution of allergic respiratory inflammation is poorly defined despite the commonality of their presence and in some cases their use as a biomarker for disease severity and/or symptom control. However, this ambiguity belies the wealth of insights that have recently been gained through the use of eosinophil-deficient/attenuated strains of mice that have demonstrated novel immunoregulatory and remodelling/repair functions for these cells in the lung following allergen provocation. Specifically, studies of eosinophil-deficient mice suggest that eosinophils contribute to events occurring in the lungs following allergen provocation at several key moments: (i) the initiating phase of events leading to Th2-polarized pulmonary inflammation, (ii) the suppression Th1/Th17 pathways in lung-draining lymph nodes, (iii) the recruitment of effector Th2 T cells to the lung, and finally, (iv) mechanisms of inflammatory resolution that re-establish pulmonary homoeostasis. These suggested functions have recently been confirmed and expanded upon using allergen provocation of an inducible eosinophil-deficient strain of mice (iPHIL) that demonstrated an eosinophil-dependent mechanism(s) leading to Th2 dominated immune responses in the presence of eosinophils in contrast to neutrophilic as well as mixed Th1/Th17/Th2 variant phenotypes in the absence of eosinophils. These findings highlighted that eosinophils are not exclusively downstream mediators controlled by T cells, dendritic cells (DC) and/or innate lymphocytic cells (ILC2). Instead, eosinophils appear to be more aptly described as significant contributors in complex interrelated pathways that lead to pulmonary inflammation and subsequently promote resolution and the re-establishment of homoeostatic baseline. In this review, we summarize and put into the context the evolving hypotheses that are now expanding our understanding of the roles eosinophils likely have in the lung following allergen provocation.

Conditional IL-4/IL-13-deficient mice reveal a critical role of innate immune cells for protective immunity against gastrointestinal helminths

Approximately one-third of the world population is infected with gastrointestinal helminths. Studies in mouse models have demonstrated that the cytokines interleukin (IL)-4 and IL-13 are essential for worm expulsion, but the critical cellular source of these cytokines is poorly defined. Here, we compared the immune response to Nippostrongylus brasiliensis in wild-type, T cell-specific IL-4/IL-13-deficient and general IL-4/IL-13-deficient mice. We show that T cell-derived IL-4/IL-13 promoted T helper 2 (Th2) polarization in a paracrine manner, differentiation of alternatively activated macrophages, and tissue recruitment of innate effector cells. However, innate IL-4/IL-13 played the critical role for induction of goblet cell hyperplasia and secretion of effector molecules like Mucin5ac and RELMβ in the small intestine. Surprisingly, T cell-specific IL-4/IL-13-deficient and wild-type mice cleared the parasite with comparable efficiency, whereas IL-4/IL-13-deficient mice showed impaired expulsion. These findings demonstrate that IL-4/IL-13 produced by cells of the innate immune system is required and sufficient to initiate effective type 2 immune responses resulting in protective immunity against N. brasiliensis.

Paradigm shifts in mast cell and basophil biology and function: an emerging view of immune regulation in health and disease

The physiological role of the mast cell and basophil has for many years remained enigmatic. In this chapter we briefly summarize some of the more recent studies that shed new light on the role of mast cells and basophils in health and disease. What we gain from these studies is a new appreciation for mast cells and basophils as sentinels in host defense and a further understanding that dysregulation of mast cell and basophil function can be a component of various diseases other than allergies. Perhaps, the most important insight reaped from this work is the increasing awareness that mast cells and basophils can function as immunoregulatory cells that modulate the immune response in health and disease. Collectively, the recent knowledge provides new challenges and opportunities towards the development of novel therapeutic strategies to augment host protection and modify disease through manipulation of mast cell and basophil function.

The future of biologics: applications for food allergy

Allergic diseases affect millions worldwide, with growing evidence of an increase in allergy occurrence over the past few decades. Current treatments for allergy include corticosteroids to reduce inflammation and allergen immunotherapy; however, some subjects experience treatment-resistant inflammation or adverse reactions to these treatments, and there are currently no approved therapeutics for the treatment of food allergy. There is a dire need for new therapeutic approaches for patients with poorly controlled atopic diseases and a need to improve the safety and effectiveness of allergen immunotherapy. Improved understanding of allergy through animal models and clinical trials has unveiled potential targets for new therapies, leading to the development of several biologics to treat allergic diseases. This review focuses on the mechanisms that contribute to allergy, with an emphasis on future targets for biologics for the treatment of food allergy. These biologics include immunotherapy with novel anti-IgE antibodies and analogs, small-molecule inhibitors of cell signaling, anti-type 2 cytokine mAbs, and TH1-promoting adjuvants.

Type 2 inflammation in asthma--present in most, absent in many

Asthma is one of the most common chronic immunological diseases in humans, affecting people from childhood to old age. Progress in treating asthma has been relatively slow and treatment guidelines have mostly recommended empirical approaches on the basis of clinical measures of disease severity rather than on the basis of the underlying mechanisms of pathogenesis. An important molecular mechanism of asthma is type 2 inflammation, which occurs in many but not all patients. In this Opinion article, I explore the role of type 2 inflammation in asthma, including lessons learnt from clinical trials of inhibitors of type 2 inflammation. I consider how dichotomizing asthma according to levels of type 2 inflammation--into 'T helper 2 (TH2)-high' and 'TH2-low' subtypes (endotypes)--has shaped our thinking about the pathobiology of asthma and has generated new interest in understanding the mechanisms of disease that are independent of type 2 inflammation.

Type 2 inflammation in asthma--present in most, absent in many

Asthma is one of the most common chronic immunological diseases in humans, affecting people from childhood to old age. Progress in treating asthma has been relatively slow and treatment guidelines have mostly recommended empirical approaches on the basis of clinical measures of disease severity rather than on the basis of the underlying mechanisms of pathogenesis. An important molecular mechanism of asthma is type 2 inflammation, which occurs in many but not all patients. In this Opinion article, I explore the role of type 2 inflammation in asthma, including lessons learnt from clinical trials of inhibitors of type 2 inflammation. I consider how dichotomizing asthma according to levels of type 2 inflammation--into 'T helper 2 (TH2)-high' and 'TH2-low' subtypes (endotypes)--has shaped our thinking about the pathobiology of asthma and has generated new interest in understanding the mechanisms of disease that are independent of type 2 inflammation.

Development, differentiation, and diversity of innate lymphoid cells

Recent years have witnessed the discovery of an unprecedented complexity in innate lymphocyte lineages, now collectively referred to as innate lymphoid cells (ILCs). ILCs are preferentially located at barrier surfaces and are important for protection against pathogens and for the maintenance of organ homeostasis. Inappropriate activation of ILCs has been linked to the pathogenesis of inflammatory and autoimmune disorders. Recent evidence suggests that ILCs can be grouped into two separate lineages, cytotoxic ILCs represented by conventional natural killer (cNK) cells and cytokine-producing helper-like ILCs (i.e., ILC1s, ILC2s, ILC3s). We will focus here on current work in humans and mice that has identified core transcriptional circuitry required for the commitment of lymphoid progenitors to the ILC lineage. The striking similarities in transcriptional control of ILC and T cell lineages reveal important insights into the evolution of transcriptional programs required to protect multicellular organisms against infections and to fortify barrier surfaces.

The role of ILC2 in pathology of type 2 inflammatory diseases

Group 2 innate lymphoid cells (ILC2) which are strategically interspersed throughout barrier surfaces are important regulators of type 2 immune reactions, particularly against helminthic parasites. ILC2 are also involved in tissue homeostasis and repair. Studies in a variety of animal models have demonstrated that when dysregulated or chronically activated, ILC2 can contribute to the pathology of allergic inflammatory diseases such as allergic asthma and atopic dermatitis. Here we discuss new findings of the cross talk of ILC2 with other hematopoietic cells, in particular T cells, and review recent information on the role of ILC2 in type 2 inflammatory diseases.

Eosinophil secretion of granule-derived cytokines

Eosinophils are tissue-dwelling leukocytes, present in the thymus, and gastrointestinal and genitourinary tracts of healthy individuals at baseline, and recruited, often in large numbers, to allergic inflammatory foci and sites of active tissue repair. The biological significance of eosinophils is vast and varied. In health, eosinophils support uterine and mammary gland development, and maintain bone marrow plasma cells and adipose tissue alternatively activated macrophages, while in response to tissue insult eosinophils function as inflammatory effector cells, and, in the wake of an inflammatory response, promote tissue regeneration, and wound healing. One common mechanism driving many of the diverse eosinophil functions is the regulated and differential secretion of a vast array of eosinophil-derived cytokines. Eosinophils are distinguished from most other leukocytes in that many, if not all, of the over three dozen eosinophil-derived cytokines are pre-synthesized and stored within intracellular granules, poised for very rapid, stimulus-induced secretion. Eosinophils engaged in cytokine secretion in situ utilize distinct pathways of cytokine release that include classical exocytosis, whereby granules themselves fuse with the plasma membrane and release their entire contents extracellularly; piecemeal degranulation, whereby granule-derived cytokines are selectively mobilized into vesicles that emerge from granules, traverse the cytoplasm and fuse with the plasma membrane to release discrete packets of cytokines; and eosinophil cytolysis, whereby intact granules are extruded from eosinophils, and deposited within tissues. In this latter scenario, extracellular granules can themselves function as stimulus-responsive secretory-competent organelles within the tissue. Here, we review the distinctive processes of differential secretion of eosinophil granule-derived cytokines.

MHCII-mediated dialog between group 2 innate lymphoid cells and CD4(+) T cells potentiates type 2 immunity and promotes parasitic helminth expulsion

Group 2 innate lymphoid cells (ILC2s) release interleukin-13 (IL-13) during protective immunity to helminth infection and detrimentally during allergy and asthma. Using two mouse models to deplete ILC2s in vivo, we demonstrate that T helper 2 (Th2) cell responses are impaired in the absence of ILC2s. We show that MHCII-expressing ILC2s interact with antigen-specific T cells to instigate a dialog in which IL-2 production from T cells promotes ILC2 proliferation and IL-13 production. Deletion of MHCII renders IL-13-expressing ILC2s incapable of efficiently inducing Nippostrongylus brasiliensis expulsion. Thus, during transition to adaptive T cell-mediated immunity, the ILC2 and T cell crosstalk contributes to their mutual maintenance, expansion and cytokine production. This interaction appears to augment dendritic-cell-induced T cell activation and identifies a previously unappreciated pathway in the regulation of type-2 immunity.

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Genetic ablation of ILC2s impairs type-2 immunity

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MHCII-expressing ILC2s potentiate Th2 responses

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IL-2 from T cells promotes ILC2 proliferation and expression of type-2 cytokines

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MHCII and IL-13 expression by ILC2s is important for N. brasiliensis expulsion

Host protective roles of type 2 immunity: parasite killing and tissue repair, flip sides of the same coin

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Type 2 immunity is associated with both helminth infection and responses to injury.

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Pathways involved in tissue repair and helminth immunity overlap.

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The IL-4Rα is central to accelerating both repair and helminth control.

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Adaptive immunity contributes to more rapid wound repair.

Metazoan parasites typically induce a type 2 immune response, characterized by T helper 2 (Th2) cells that produce the cytokines IL-4, IL-5 and IL-13 among others. The type 2 response is host protective, reducing the number of parasites either through direct killing in the tissues, or expulsion from the intestine. Type 2 immunity also protects the host against damage mediated by these large extracellular parasites as they migrate through the body. At the center of both the innate and adaptive type 2 immune response, is the IL-4Rα that mediates many of the key effector functions. Here we highlight the striking overlap between the molecules, cells and pathways that mediate both parasite control and tissue repair. We have proposed that adaptive Th2 immunity evolved out of our innate repair pathways to mediate both accelerated repair and parasite control in the face of continual assault from multicellular pathogens. Type 2 cytokines are involved in many aspects of mammalian physiology independent of helminth infection. Therefore understanding the evolutionary relationship between helminth killing and tissue repair should provide new insight into immune mechanisms of tissue protection in the face of physical injury.

Aberrant production of IL-13 by T cells promotes exocrinopathy in Id3 knockout mice

Elevated levels of the cytokine IL-13 has been found to be associated with autoimmune diseases, including Sjögren's Syndrome. However, whether IL-13 plays a causative role in disease development is not known and cannot be easily studied in humans. Our previous work has shown that levels of IL-13 are elevated in Id3 knockout mice, which has been established as a model for primary Sjögren's Syndrome. Here, we utilized an IL-13 reporter to determine the source of the elevated IL-13 levels observed in Id3 knockout mice and assess its contribution to SS pathology. Our results indicate that T cells, notably CD4 and γδ T cells, in Id3 knockout mice acquire IL-13 competency at an elevated rate well before disease symptoms become apparent. We also show that T cells developing early in life are more predisposed to produce IL-13. Finally, analysis of Id3 and IL-13 double deficient mice demonstrated that IL-13 plays an essential role in the deterioration of gland function. Our study provides crucial genetic evidence that enhanced IL-13 production by T cells can play a causative role in the exocrinopathy observed in Id3 knockout mice.

Systemic impact of intestinal helminth infections

In this review, we examine the evidence that intestinal helminths can control harmful inflammatory responses and promote homeostasis by triggering systemic immune responses. Induction of separable components of immunity by helminths, which includes type 2 and immune regulatory responses, can both contribute toward the reduction in harmful type 1 immune responses that drive certain inflammatory diseases. Despite inducing type 2 responses, intestinal helminths may also downregulate harmful type 2 immune responses including allergic responses. We consider the possibility that intestinal helminth infection may indirectly affect inflammation by influencing the composition of the intestinal microbiome. Taken together, the studies reviewed herein suggest that intestinal helminth-induced responses have potent systemic effects on the immune system, raising the possibility that whole parasites or specific molecules produced by these metazoans may be an important resource for the development of future immunotherapies to control inflammatory diseases.

Eosinophils are important for protection, immunoregulation and pathology during infection with nematode microfilariae

Eosinophil responses typify both allergic and parasitic helminth disease. In helminthic disease, the role of eosinophils can be both protective in immune responses and destructive in pathological responses. To investigate whether eosinophils are involved in both protection and pathology during filarial nematode infection, we explored the role of eosinophils and their granule proteins, eosinophil peroxidase (EPO) and major basic protein-1 (MBP-1), during infection with Brugia malayi microfilariae. Using eosinophil-deficient mice (PHIL), we further clarify the role of eosinophils in clearance of microfilariae during primary, but not challenge infection in vivo. Deletion of EPO or MBP-1 alone was insufficient to abrogate parasite clearance suggesting that either these molecules are redundant or eosinophils act indirectly in parasite clearance via augmentation of other protective responses. Absence of eosinophils increased mast cell recruitment, but not other cell types, into the broncho-alveolar lavage fluid during challenge infection. In addition absence of eosinophils or EPO alone, augmented parasite-induced IgE responses, as measured by ELISA, demonstrating that eosinophils are involved in regulation of IgE. Whole body plethysmography indicated that nematode-induced changes in airway physiology were reduced in challenge infection in the absence of eosinophils and also during primary infection in the absence of EPO alone. However lack of eosinophils or MBP-1 actually increased goblet cell mucus production. We did not find any major differences in cytokine responses in the absence of eosinophils, EPO or MBP-1. These results reveal that eosinophils actively participate in regulation of IgE and goblet cell mucus production via granule secretion during nematode-induced pathology and highlight their importance both as effector cells, as damage-inducing cells and as supervisory cells that shape both innate and adaptive immunity.

Eosinophil recruitment is a classic characteristic of both allergic and parasitic helminth diseases. Elucidation of the role of eosinophils in these diseases is of pivotal importance for understanding the mechanisms of protection and the development of pathology. In the last few years, the part played by eosinophils in helminth-defence has been dissected using in vivo models and their importance in protection has been shown to be highly specific to the host-parasite combination. This study dissects the role of eosinophils during infection with the human lymphatic filarial parasite, Brugia malayi, which causes the major neglected tropical disease, lymphatic filariasis. In particular, we study the role of the eosinophil as a double–edged sword in generating both protection and pathology. We definitively confirm the importance of eosinophils in protection against B. malayi microfilariae and show that protection is not mediated by release of the eosinophil granule proteins, major basic protein or eosinophil peroxidase alone. Overall, we reveal that during an infection with B. malayi microfilariae, eosinophils are critical for primary protective responses. However, eosinophils contribute to nematode-induced lung dysfunction, while additionally, eosinophil granules are important negative regulators of parasite-induced lung inflammatory and some adaptive immune responses.

Interleukin-13-mediated paneth cell degranulation and antimicrobial peptide release

Paneth cell-derived enteric antimicrobial peptides significantly contribute to antibacterial host defense and host-microbial homeostasis. Regulation occurs by enzymatic processing and release into the small intestinal lumen, but the stimuli involved are incompletely understood. Here, the capacity of various microbial and immune stimuli to induce antimicrobial peptide release from small intestinal tissue was systematically evaluated using antibacterial activity testing, immunostaining for Paneth cell granules and mass spectrometry. We confirmed the stimulatory activity of the muscarinic receptor agonist carbachol and the nucleotide-binding oligomerization domain ligand muramyl dipeptide. In contrast, no release of antibacterial activity was noted after treatment with the Toll-like receptor ligands poly(I:C), lipopolysaccharide or CpG, and the cytokines interleukin (IL)-15, IL-22, IL-28 and interferon-γ. Rapid Paneth cell degranulation and antimicrobial activity release, however, was observed after stimulation with the endogenous mediators IL-4 and IL-13. This process required phosphatidylinositol 3-kinase and was associated with protein kinase B phosphorylation in Paneth cells. Flow cytometric analysis confirmed expression of the IL-13 receptor α1 on isolated Paneth cells. Our findings identify a novel role of IL-13 as inducer of Paneth cell degranulation and enteric antimicrobial peptide release. IL-13 may thus contribute to mucosal antimicrobial host defense and host microbial homeostasis.

Inducible deletion of CD28 prior to secondary nippostrongylus brasiliensis infection impairs worm expulsion and recall of protective memory CD4⁺ T cell responses

IL-13 driven Th2 immunity is indispensable for host protection against infection with the gastrointestinal nematode Nippostronglus brasiliensis. Disruption of CD28 mediated costimulation impairs development of adequate Th2 immunity, showing an importance for CD28 during the initiation of an immune response against this pathogen. In this study, we used global CD28^−/− mice and a recently established mouse model that allows for inducible deletion of the cd28 gene by oral administration of tamoxifen (CD28^−/loxCre^+/−+TM) to resolve the controversy surrounding the requirement of CD28 costimulation for recall of protective memory responses against pathogenic infections. Following primary infection with N. brasiliensis, CD28^−/− mice had delayed expulsion of adult worms in the small intestine compared to wild-type C57BL/6 mice that cleared the infection by day 9 post-infection. Delayed expulsion was associated with reduced production of IL-13 and reduced serum levels of antigen specific IgG1 and total IgE. Interestingly, abrogation of CD28 costimulation in CD28^−/loxCre^+/− mice by oral administration of tamoxifen prior to secondary infection with N. brasiliensis resulted in impaired worm expulsion, similarly to infected CD28^−/− mice. This was associated with reduced production of the Th2 cytokines IL-13 and IL-4, diminished serum titres of antigen specific IgG1 and total IgE and a reduced CXCR5⁺ T_FH cell population. Furthermore, total number of CD4⁺ T cells and B220⁺ B cells secreting Th1 and Th2 cytokines were significantly reduced in CD28^−/− mice and tamoxifen treated CD28^−/loxCre^+/− mice compared to C57BL/6 mice. Importantly, interfering with CD28 costimulatory signalling before re-infection impaired the recruitment and/or expansion of central and effector memory CD4⁺ T cells and follicular B cells to the draining lymph node of tamoxifen treated CD28^−/loxCre^+/− mice. Therefore, it can be concluded that CD28 costimulation is essential for conferring host protection during secondary N. brasiliensis infection.

CD28 is an important costimulatory molecule, involved in the activation of naive T cells, enhancing cytokine production, preventing T cell anergy and apoptosis. Furthermore, CD28 plays a crucial role in the organisation of secondary lymphoid tissue by assisting in the recruitment of T cells into the B cell follicles, thus promoting germinal center formation, isotype switching and B cell maturation. The requirement of CD28 costimulatory signalling during recall of memory responses against infections has remained controversial. Hence, here we utilised a mouse model that allowed for inducible deletion of the cd28 gene (CD28^−/loxCre^+/−) by oral administration of tamoxifen to resolve this controversy. CD28^−/− mice and mice given tamoxifen prior to secondary infection failed to expel adult N. brasiliensis worms. This was related to reduced production of the Th2 cytokines IL-13 and IL-4, diminished type 2 antibody titres, and a reduced number of memory CD4⁺ T cells. In summary, CD28 is crucial for protection against N. brasiliensis secondary infection and plays a key role in the recruitment of T_FH cells, memory CD4⁺ T cells and follicular B cells.

IL-9-mediated survival of type 2 innate lymphoid cells promotes damage control in helminth-induced lung inflammation

IL-9 acts as an autocrine amplifier of type 2 innate lymphoid cell function to promote tissue repair in the recovery phase of helminth-induced lung infection.

IL-9 fate reporter mice established type 2 innate lymphoid cells (ILC2s) as major producers of this cytokine in vivo. Here we focus on the role of IL-9 and ILC2s during the lung stage of infection with Nippostrongylus brasiliensis, which results in substantial tissue damage. IL-9 receptor (IL-9R)–deficient mice displayed reduced numbers of ILC2s in the lung after infection, resulting in impaired IL-5, IL-13, and amphiregulin levels, despite undiminished numbers of Th2 cells. As a consequence, the restoration of tissue integrity and lung function was strongly impaired in the absence of IL-9 signaling. ILC2s, in contrast to Th2 cells, expressed high levels of the IL-9R, and IL-9 signaling was crucial for the survival of activated ILC2s in vitro. Furthermore, ILC2s in the lungs of infected mice required the IL-9R to up-regulate the antiapoptotic protein BCL-3 in vivo. This highlights a unique role for IL-9 as an autocrine amplifier of ILC2 function, promoting tissue repair in the recovery phase after helminth-induced lung inflammation.

IL-4Rα-associated antigen processing by B cells promotes immunity in Nippostrongylus brasiliensis infection

In this study, B cell function in protective T_H2 immunity against N. brasiliensis infection was investigated. Protection against secondary infection depended on IL-4Rα and IL-13; but not IL-4. Protection did not associate with parasite specific antibody responses. Re-infection of B cell-specific IL-4Rα^−/− mice resulted in increased worm burdens compared to control mice, despite their equivalent capacity to control primary infection. Impaired protection correlated with reduced lymphocyte IL-13 production and B cell MHC class II and CD86 surface expression. Adoptive transfer of in vivo N. brasiliensis primed IL-4Rα expressing B cells into naïve BALB/c mice, but not IL-4Rα or IL-13 deficient B cells, conferred protection against primary N. brasiliensis infection. This protection required MHC class II compatibility on B cells suggesting cognate interactions by B cells with CD4⁺ T cells were important to co-ordinate immunity. Furthermore, the rapid nature of these protective effects by B cells suggested non-BCR mediated mechanisms, such as via Toll Like Receptors, was involved, and this was supported by transfer experiments using antigen pulsed Myd88^−/− B cells. These data suggest TLR dependent antigen processing by IL-4Rα-responsive B cells producing IL-13 contribute significantly to CD4⁺ T cell-mediated protective immunity against N. brasiliensis infection.

Parasitic nematode infections are an extremely important global public health problem. Infections by hookworms and roundworms for example cause anemia, widespread developmental problems and devalued immunity against bacterial infections such as salmonella and tuberculosis. Although treatable with drugs, parasitic nematode re-infections occur as humans do not develop protective immunity. Ultimately, the public health burden caused by these infections will be best controlled by the development of vaccines against nematode infections. For these to be effective, it is important to understand how the various components of the immune system can respond to infection. In this study, we show that B cells, which typically protect against infection by producing antibodies, can also protect against an experimental hookworm like nematode infection by additional mechanisms. This form of protection instead depended on B cells producing cytokines associated with parasitic nematode expulsion and also by providing T cells with specific instruction. Together, these B cell driven responses lead to a rapid resolution of the infection. These important findings indicate that vaccination strategies against nematode parasites such as hookworms need to understand immune responses other than antibody to be optimally protective.

Control of T helper 2 responses by transcription factor IRF4-dependent dendritic cells

CD4⁺ T cell differentiation is regulated by specialized antigen-presenting cells. Dendritic cells (DCs) produce cytokines that promote naive CD4⁺ T cell differentiation into T helper 1 (Th1), Th17, and inducible T regulatory (iTreg) cells. However, the initiation of Th2 cell responses remains poorly understood, although it is likely that more than one mechanism might be involved. Here we have defined a specific DC subset that is involved in Th2 cell differentiation in vivo in response to a protease allergen, as well as infection with Nippostrongylus brasiliensis. We have demonstrated that this subset is controlled by the transcription factor interferon regulatory factor 4 (IRF4), which is required for their differentiation and Th2 cell-inducing function. IRF4 is known to control Th2 cell differentiation and Th2 cell-associated suppressing function in Treg cells. Our finding suggests that IRF4 also plays a role in DCs where it controls the initiation of Th2 cell responses.

IL-33 markedly activates murine eosinophils by an NF-κB-dependent mechanism differentially dependent upon an IL-4-driven autoinflammatory loop

Eosinophils are major effector cells in type 2 inflammatory responses and become activated in response to IL-4 and IL-33, yet the molecular mechanisms and cooperative interaction between these cytokines remain unclear. Our objective was to investigate the molecular mechanism and cooperation of IL-4 and IL-33 in eosinophil activation. Eosinophils derived from bone marrow or isolated from Il5-transgenic mice were activated in the presence of IL-4 or IL-33 for 1 or 4 h, and the transcriptome was analyzed by RNA sequencing. The candidate genes were validated by quantitative PCR and ELISA. We demonstrated that murine-cultured eosinophils respond to IL-4 and IL-33 by phosphorylation of STAT-6 and NF-κB, respectively. RNA sequence analysis of murine-cultured eosinophils indicated that IL-33 induced 519 genes, whereas IL-4 induced only 28 genes, including 19 IL-33-regulated genes. Interestingly, IL-33 induced eosinophil activation via two distinct mechanisms, IL-4 independent and IL-4 secretion/autostimulation dependent. Anti-IL-4 or anti-IL-4Rα Ab-treated cultured and mature eosinophils, as well as Il4- or Stat6-deficient cultured eosinophils, had attenuated protein secretion of a subset of IL-33-induced genes, including Retnla and Ccl17. Additionally, IL-33 induced the rapid release of preformed IL-4 protein from eosinophils by a NF-κB-dependent mechanism. However, the induction of most IL-33-regulated transcripts (e.g., Il6 and Il13) was IL-4 independent and blocked by NF-κB inhibition. In conclusion, we have identified a novel activation pathway in murine eosinophils that is induced by IL-33 and differentially dependent upon an IL-4 auto-amplification loop.

Association of IL-4-590 C>T and IL-13-1112 C>T gene polymorphisms with the susceptibility to type 2 diabetes mellitus

Background. The goal of the study is to investigate the association of IL-4-590 and IL-13-1112 genetic polymorphisms with type 2 diabetes mellitus (T2DM) in Egyptian patients. Subjects and Methods. The study included 135 cases with T2DM and 75 healthy unrelated age-matched controls from the same locality of Egypt. DNA was extracted and processed by the ARMS-PCR technique for characterization of genetic variants of IL-4-590 C>T and IL-13-1112 C>T polymorphisms. Results. Egyptian cases with T2DM showed a lower frequency of the IL-4-590 CC homozygous genotype compared to controls (10.4% versus 43.48%) with a higher CT heterozygous genotype (85.2% versus 47.8%). Similarly, cases showed a lower frequency of the IL-13-1112 CC genotype (20.7% versus 56.8%) with a higher frequency of the heterozygous IL-13-1112 CT genotype (76.3% versus 41.3%). Both polymorphisms showed significantly positive associations with T2DM in the dominant, codominant, and overdominant models of inheritance. On the other hand, comparing genotypes of subgroups related to gender, positive family history, and positive consanguinity showed a nonsignificant difference (P > 0.05). Conclusion. Heterozygous genotypes (IL-4-590 CT and IL-13-1112 CT) could be considered as risk factors, while the homozygous wild types (-590 CC and -1112 CC) might be considered protective to T2DM.

IL-4 attenuates Th1-associated chemokine expression and Th1 trafficking to inflamed tissues and limits pathogen clearance

Interleukin 4 (IL-4) plays a central role in the orchestration of Type 2 immunity. During T cell activation in the lymph node, IL-4 promotes Th2 differentiation and inhibits Th1 generation. In the inflamed tissue, IL-4 signals promote innate and adaptive Type-2 immune recruitment and effector function, positively amplifying the local Th2 response. In this study, we identify an additional negative regulatory role for IL-4 in limiting the recruitment of Th1 cells to inflamed tissues. To test IL-4 effects on inflammation subsequent to Th2 differentiation, we transiently blocked IL-4 during ongoing dermal inflammation (using anti-IL-4 mAb) and analyzed changes in gene expression. Neutralization of IL-4 led to the upregulation of a number of genes linked to Th1 trafficking, including CXCR3 chemokines, CCL5 and CCR5 and an associated increase in IFNγ, Tbet and TNFα genes. These gene expression changes correlated with increased numbers of IFNγ-producing CD4+ T cells in the inflamed dermis. Moreover, using an adoptive transfer approach to directly test the role of IL-4 in T cell trafficking to the inflamed tissues, we found IL-4 neutralization led to an early increase in Th1 cell recruitment to the inflamed dermis. These data support a model whereby IL-4 dampens Th1-chemokines at the site of inflammation limiting Th1 recruitment. To determine biological significance, we infected mice with Leishmania major, as pathogen clearance is highly dependent on IFNγ-producing CD4+ T cells at the infection site. Short-term IL-4 blockade in established L. major infection led to a significant increase in the number of IFNγ-producing CD4+ T cells in the infected ear dermis, with no change in the draining LN. Increased lymphocyte influx into the infected tissue correlated with a significant decrease in parasite number. Thus, independent of IL-4's role in the generation of immune effectors, IL-4 attenuates lymphocyte recruitment to the inflamed/infected dermis and limits pathogen clearance.

Protective and pathological roles of mast cells and basophils

Mast cells and basophils are potent effector cells of the innate immune system, and they have both beneficial and detrimental functions for the host. They are mainly implicated in pro-inflammatory responses to allergens but can also contribute to protection against pathogens. Although both cell types were identified more than 130 years ago by Paul Ehrlich, their in vivo functions remain poorly understood. The precursor cell populations that give rise to mast cells and basophils have recently been characterized and isolated. Furthermore, new genetically modified mouse strains have been developed, which enable more specific targeting of mast cells and basophils. Such advances offer new opportunities to uncover the true in vivo activities of these cells and to revisit their previously proposed effector functions.

Development and function of group 2 innate lymphoid cells

The innate lymphoid cell (ILC) family has recently expanded with the discovery of type-2 innate lymphoid cells (ILC2). These cells arise from lymphoid progenitors in the bone marrow and, under the control of the transcriptional regulators RORα and Gata3, they mature to give rise to IL-5, IL-9 and IL-13 producing ILC2. These cells are critical components of the innate immune response to parasitic worm infections and have also been implicated in the pathogenesis of asthma and allergy. Recent advances in our understanding of the molecular regulation of ILC2 development and function now present the opportunity to develop new genetic models to assess ILC2 immune function and to investigate possible therapeutic interventions.

Macrophages as IL-25/IL-33-responsive cells play an important role in the induction of type 2 immunity

Type 2 immunity is essential for host protection against nematode infection but is detrimental in allergic inflammation or asthma. There is a major research focus on the effector molecules and specific cell types involved in the initiation of type 2 immunity. Recent work has implicated an important role of epithelial-derived cytokines, IL-25 and IL-33, acting on innate immune cells that are believed to be the initial sources of type 2 cytokines IL-4/IL-5/IL-13. The identities of the cell types that mediate the effects of IL-25/IL-33, however, remain to be fully elucidated. In the present study, we demonstrate that macrophages as IL-25/IL-33-responsive cells play an important role in inducing type 2 immunity using both in vitro and in vivo approaches. Macrophages produced type 2 cytokines IL-5 and IL-13 in response to the stimulation of IL-25/IL-33 in vitro, or were the IL-13-producing cells in mice administrated with exogenous IL-33 or infected with Heligmosomoides bakeri. In addition, IL-33 induced alternative activation of macrophages primarily through autocrine IL-13 activating the IL-4Rα-STAT6 pathway. Moreover, depletion of macrophages attenuated the IL-25/IL-33-induced type 2 immunity in mice, while adoptive transfer of IL-33-activated macrophages into mice with a chronic Heligmosomoides bakeri infection induced worm expulsion accompanied by a potent type 2 protective immune response. Thus, macrophages represent a unique population of the innate immune cells pivotal to type 2 immunity and a potential therapeutic target in controlling type 2 immunity-mediated inflammatory pathologies.

Development of CD4 T Cell Dependent Immunity Against N. brasiliensis Infection

Of all the microbial infections relevant to mammals the relationship between parasitic worms and what constitutes and regulates a host protective immune response is perhaps the most complex and evolved. Nippostrongylus brasiliensis is a tissue migrating parasitic roundworm of rodents that exemplifies many of the salient features of parasitic worm infection, including parasite development through sequential larval stages as it migrates through specific tissue sites. Immune competent hosts respond to infection by N. brasiliensis with a rapid and selective development of a profound Th2 immune response that appears able to confer life long protective immunity against reinfection. This review details how the lung can be the site of migrating nematode immune killing and the gut a site of rapid immune mediated clearance of worms. Furthermore it appears that N. brasiliensis induced responses in the lung are sufficient for conferring immunity in lung and gut while infection of the gut only confers immunity in the gut. This review also covers the role of IL-4, STAT6, and the innate cytokines IL-25, IL-33, and thymic stromal lymphopoietin in the generation of CD4-mediated immunity against N. brasiliensis reinfection and discusses what cytokines might be involved in mediated killing or expulsion of helminth parasites.